Since WebKit is open source, can't you just submit bugfixes for the bugs that are parallelism bottlenecks? Seems like that would make a lot more sense than coding another engine to accommodate those bugs. If it truly is a bug, there shouldn't be any problem with submitting a bugfix and getting it accepted.
Since you're working on FireFox, are there any examples of WebKit "bugs" that prevent parallelism that you could not fix yourself? Does Mozilla have a team of WebKit engineers whose sole job is to fix WebKit so that those monoculture problems are mitigated and don't become a problem for other browser engines? At the end of the day, all you guys need to defend is the interface, not the implementation. Fixing each other's engines before bugs become features seems like a good way to accomplish this. ref: http://xkcd.com/1172/
The most important abstraction to fix isn't even a WebKit abstraction, it's a W3C abstraction. Everything was doomed from the get go because of the one-to-one relationship between the window and the document. I agree with Kay here about TBL & Co being shortsighted in what the web could have become if a richer interactive experience instead of a document based experience had been considered from inception.
Look at Twitter. That's not a document. That's an application. Each tweet in the interface is a document. Every tweet in that feed is a document that has been "transcluded" into the app Twitter built on top of a document. There needs to be a standard way to "transclude" documents with reference URL that allows interactivity. The fact that the only hyperlinking option we have today is the <a> tag is unfortunate. There need to be more ways of hyperlinking than an <a> tag. You need to be able to window directly to a document or document fragment at a different URL. The #hashanchors aren't sufficient, since they only describe a beginning, not an end to the fragment being excerpt. iFrames kind of provide an alternative, but this was never explored properly. The host app should also be able to provide a cached copy of the contents of any sub-document for performance and to guarantee that a copy of the referred document is always available in the parent context.
edit: downvote? srsly? without a response? downvoting is for comments that don't contribute to the conversation, not for comments you simply don't agree with.